This invention relates to electronic article surveillance (EAS) systems and, in particular, to apparatus for operating such systems and to antennas to be used in such systems.
U.S. Pat. No. 4,686,513, assigned to the same assignee hereof, discloses an EAS system in which a tag in an interrogation zone is subjected to coded messages which are transmitted into the zone by an antenna driven by a transmitter. These coded messages contain commands for the tag and a variety of commands can be used to invoke various actions by the tag. For example, one command may instruct the tag to transmit a coded alarm message which can be received by the antenna and coupled to a receiver. The receiver upon receipt of the alarm message can then activate an alarm to indicate that the tag is present in the zone.
The '513 patent also discloses a number of techniques for isolating the interrogation zone so that tags outside the zone are not subjected to the transmitted coded messages. One technique described is to utilize an antenna arrangement comprising two facing antennas which are turned on and off alternately and each of which transmits half of a coded message throughout the entire interrogation zone. In this way, tags within the interrogation zone receive both halves of the coded message, i.e., the entire message and, therefore, can respond accordingly. Tags outside the zone, however, only receive one or the other half of the coded message and, hence, will not respond.
While the aforesaid technique provides desirable isolation of the interrogation zone, it also requires that each antenna operate at a power level sufficient to transmit its half message over the entire zone. This power requirement is a decided disadvantage and prevents the technique from being used in many applications.
Most systems of the '513 patent type in use today employ a single antenna which transmits the entire coded message into the interrogation zone. When using such a single antenna, isolation of the zone and limiting the power used are generally realized by positioning the antenna appropriately and by limiting the size of the zone.
However, recent demands to employ the '513 patent system with interrogation zones of increased size, have spurred efforts to modify the system to meet these demands. One suggested modification has been to utilize two opposing loop antennas to simultaneously transmit the same coded message in its entirety into complementary parts of the interrogation zone. This has the advantage of limiting the power required for each antenna which also tends to limit the transmission outside the zone, including that occurring in the so-called "backfield".
With such a two loop system, in order to ensure that the entire interrogation zone is covered, a considerable degree of overlap of the zone parts covered by the transmissions from the two antennas occurs. In the overlap region, which is usually at the center of the interrogation zone, the transmissions from the two antennas tend to cancel each other. The result is a null zone which is devoid of coded message content. As can be appreciated, the presence of such a null zone is undesirable, since tags passing through the null zone will not be able to receive and respond to the transmitted messages and will go undetected.
Also, the transmissions from the proposed two loop antennas are not easily confinable to the desired zone parts and the loop antennas are themselves subject to disturbances from outside the zone. Undesired coupling of the transmissions from the loop antennas to surrounding structures such as, for example, metal conduits, support beams and door frames, additionally undesirably enlarges the field outside the zone. This is especially so for the field adjacent the lower part of the antennas, since the antennas are usually mounted in much closer proximity to the floor than to the ceiling. Finally, the proposed antennas provide a limited transmission field in the vertical direction which makes it difficult for tags positioned horizontally to respond to the antennas.
Various multiple loop, symmetrical antenna structures are known which partially compensate for some of these effects. These known antenna structures tend to compensate primarily for so-called "far field" effects, i.e., tend to enhance cancellation of antenna transmissions far from the antennas and to promote cancellation of disturbances in the antennas which originate far from the antennas (see, for example, U.S. Pat. Nos. 4,243,980, 4,260,990, 4,751,516 and 4,135,183). However, these known antennas do not also compensate for the coupling and other undesirable effects discussed above with respect to the two loop antenna system.
It is, therefore, an object of the present invention to provide an EAS system of the '513 patent type and an antenna assembly which overcome the above-discussed disadvantages.
It is also an object of the present invention to provide an EAS system of the '513 patent type and an antenna assembly in which the power requirements are lessened as compared to the '513 patent half message transmitter switching pattern, while null zones are avoided.
It is yet a further object of the present invention to provide an EAS system of the '513 patent type and an antenna assembly in which coupling of the transmitted field to adjacent structures is lessened and uniformity of the transmitted field is promoted.
It is still a further object of the present invention to provide an EAS system of the '513 patent type and an antenna assembly in which the strength of the field components in the vertical direction for the antenna transmissions is enhanced.